Previous: ILAENV-ILAENVSET, Up: LAPACK
Some LAPACK eigensystem routines (namely CHEEVR, DSTEVR, DSYEVR, SSTEVR, SSYEVR, ZHEEVR) are able to take advantage of a faster algorithm when the full eigenspectrum is requested on machines which conform to the IEEE-754 floating point standard [14].
Normal execution of the faster algorithm (implemented by LAPACK routines SSTEGR and DSTEGR, which are called by the routines mentioned above) may create NaNs and infinities and hence may abort due to a floating point exception in environments which do not handle NaNs and infinities in the IEEE standard default manner. This may depend upon the compiler flags used to compile and link the main program.
The LAPACK routine ILAENV, called with ISPEC = 10 or 11, states whether or not NaNs or infinities respectively will cause a trap. In ACML, by default ILAENV assumes that NaNs and infinities cause traps, even if this reduces the performance of the eigensystem routines. This is because it is not possible in general to reliably check whether they do trap or not at run-time. The intention is to ensure that these routines always function correctly, irrespective of how the main program calling ACML is compiled.
However, if your main program is compiled in such a way that NaNs and infinities do not cause traps, the ACML-specific routine ILAENVSET (see ILAENV-ILAENVSET) may be used to override the default operative mode of ILAENV, and allow the xxxEVR routines to use the faster xSTEGR algorithm when calculating the full eigenspectrum. When used for this purpose, ILAENVSET should be called as follows:
CALL ILAENVSET(10,'X','X',0,0,0,0,1,INFO)
CALL ILAENVSET(11,'X','X',0,0,0,0,1,INFO)
(or the C equivalent).
It is important to note that if you use ILAENVSET in this way before calling an xxxEVR routine, but your program does trap on IEEE exceptions, then there is a chance that your program will terminate unexpectedly. You should consult the documentation for the compiler you are using to find out whether there are compiler flags controlling this.